Feature Request: Bring back tee topology implementation

[arthurschreiber]

Feature Description

We're planning on splitting up an existing etcd cluster holding multiple keyspaces into two separate clusters. According to the topology documentation, there should be a tee topology implementation that would allow doing this. See https://vitess.io/docs/21.0/reference/features/topology-service/#migration-using-the-tee-implementation

Unforutnately, it looks like the code for this implementation was deleted in #15515.

Use Case(s)

Breaking up an existing etcd topology cluster into two.